Flame retardant and flame resistant (FR) fabrics are crucial in both military and non-military environments. Firefighters, race car drivers, and petro-chemical workers are just a few of the non-military groups that benefit from the added protection of flame retardant fabrics. However, today the true benefit of flame retardant fabrics lies with the military. In addition to the unforgiving surroundings that military troops must operate in, the advent of unconventional modern warfare creates an even more hostile environment. Specifically, the use of improvised explosive devices (“IEDs”) to immobilize large convoys of soldiers makes individual troop protection critically important.
In addition to ballistic fabrics and body armor, flame retardant fabrics serve a crucial role in protecting soldiers from IEDs. IEDs are constructed of numerous materials (e.g. high-explosive charges, flammable liquids, shrapnel, etc.), some acting as projectiles and others acting as incendiaries upon detonation. Thus, military fabrics must be of varied construction to handle the multitude of threats from an IED.
There are basically two types of flame retardant fabrics used in protective clothing: (1) Fabrics made from flame retardant organic fibers (e.g. aramid, flame retardant rayon, polybenzimidazole, modacrylic etc.); and (2) Flame retardant fabrics made from conventional materials (e.g. cotton) that have been post treated to impart flame retardancy. Nomex® and Kevlar® aromatic polyamides are among the most common types of flame retardant synthetic fibers. These are made by solution spinning a meta- or para-aromatic polyamide polymer into fiber. Aromatic polyamides do not melt under extreme heat, are naturally flame retardant, but must be solution spun. Unfortunately, Nomex® and KEVLAR® are not very comfortable and are difficult and expensive to produce.
Another fiber used in protective clothing is modacrylic which is fiber comprising 30 to 70 parts by mass of acrylonitrile and 70 to 30 parts by mass of a monomer such as a halogen-containing vinylidene monomer and/or a halogen-containing vinyl monomer. Commercial examples include PROTEX® C and PROTEX® M fibers manufactured by Kaneka. At an approximate 1:1 blend ratio, modacrylic fibers are known to impart flame resistance properties to fabrics comprising non-FR treated cellulosic fibers such as cotton and lyocell. Examples can be found in EP1498522 and WO2008027454.
Cellulose fibers such as acetate, rayon, lyocell, and cotton can be rendered flame resistant by incorporating phosphorus-nitrogen additives at fiber spinning or at fabric finishing.
The mechanisms for flame resistance performance of both modacrylic and flame resistant cellulose rely on gases emitted from the fibers which dilute, cool, or chemically neutralize flammable gases (vapor phase action) and which form intumescent char barriers (condensed phase action).
Post-treatment flame retardants are applied to fabrics and can be broken down into two basic categories: (1) Durable flame retardants; and (2) Non-durable flame retardants. For protective clothing, the treatment must withstand laundering, so only durable treatments are selected. Today, most often, durable flame retardant chemistry relies on phosphorus-based FR agents and chemicals or resins to fix the FR agents to the fibers.
One polymer fiber that has been widely studied because of its processability and strength is nylon 6,6 fiber. A small amount—about 12%—of aliphatic nylon fibers can be blended with cotton in a yarn to produce a fabric; where the yarn and/or the fabric made therefrom is chemically treated to produce a flame retardant fabric. Because cotton is the major fiber component, this fabric may be called “FR cotton” fabric. Nylon fibers impart superior wear resistance to FR cotton fabrics and garments. However, because nylon is melt processable (i.e. thermoplastic) and offers no inherent flame resistance, the quantity of nylon fiber in an FR fabric, such as an FR treated cotton fabric, is limited. Attempts to increase nylon fiber content by chemical modification of aliphatic nylon fibers or development of new flame retarding fabric treatments have been unsuccessful.